Microbially Induced Corrosion

Microbes
are among the oldest creatures to inhabit the earth and there are microbes that
will attack metals including iron and steel.

There
are a number of species of microbes that attack metals in the marine world but
there are four that the small craft marine surveyor is most likely to meet.

The
oldest known of these was first reported by Dr. Ehrenberg in 1836.

It
is often called the iron bacterium and was mentioned by Kenneth Barnaby in his
book ‘basic naval architecture’ in 1954. It was known to Isambard Kingdom
Brunell.

It
is called Gallionella Ferruginea.

Gallionella
Ferruginea characteristically leaves a fairly shallow pit of approximately
oval shape and a ‘rusticle’ made of ferrous and ferric hydroxide. This is a
brown non toxic insoluble powder with black streaks. It is NOT rust though a marine surveyor known to the
author has described it in one of his reports by the curiously contradictory
name of living rust. The name ‘rusticle’ was given to the detritus by Dr. Ballard
when he found extensive amounts of the stuff on the wreck of the titanic.
It is commonly found on narrow boats and other canal barges.

Severe Gallionella attack on a narrowboat

Often
closely associated with the Gallionella species is a sulphur oxidizing
bug (SOB) called Thiobascillus Ferro-oxidans. This leaves a similar pit
to the Gallionella species but with vertical stepped sides and the flat
bottom covered with a hard white substance. The latter is tetra hydrated
ferrous sulphide and is non toxic.

Microbially induced corrosion Thiobascilluss attack on a narrowboat

There
are two further sulphur reducing bugs (SRB) of the genera Desulphotomaculum and
Desulphurovibrio. They leave the same characteristic pits but the bottom
is covered with the highly corrosive black hydrogen sulphide which, if scraped,
stinks like rotten eggs. These are not commonly found in canal waters.

If
a boat is found with evidence of microbial attack, it is necessary to deal with
it to prevent, if possible, it happening again. The whole area should be was
with copious amounts of high pressure fresh water. When dry the area affected
should be coated with stergene bleach (sodium hyperchloride) and left for
twenty four hours. Afterward another high pressure fresh water wash is
necessary followed by drying and recoating. Nevertheless, the microbes can
still live underneath nearby paint coatings.

WarningSodium hyperchloride is highly caustic and toxic. It must be treated with great care and rubber gloves, wellington boots and eye shields are ESSENTIAL.